Clean up Stopwatch a bit

[stephentoub]

• Remove unnecessary Reset call from ctor
• Remove unnecessary branch for _elapsed < 0 (Stop is now more inlineable)
• Remove some defunct comments
• Clean up style of Stop to match that of Start

Closes #66734
Related to #111829

Method	Toolchain	Mean	Ratio	Code Size	Allocated	Alloc Ratio
Time	\main\corerun.exe	38.51 ns	1.00	363 B	40 B	1.00
Time	\pr\corerun.exe	30.35 ns	0.79	156 B	-	0.00

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Diagnostics;
using System.Runtime.CompilerServices;

BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

[DisassemblyDiagnoser]
[MemoryDiagnoser(false)]
public partial class Tests
{
    [Benchmark]
    public TimeSpan Time()
    {
        Stopwatch sw = Stopwatch.StartNew();
        Nop();
        sw.Stop();
        return sw.Elapsed;
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static void Nop() { }
}

[paulbartrum]

Are the Debug.Assert(IsHighResolution); asserts still needed? These will always be true as far as I can tell.

[stephentoub]

Are the Debug.Assert(IsHighResolution); asserts still needed? These will always be true as far as I can tell.

They're not needed.

[MihaZupan]

Nice

[stephentoub]

/ba-g unrelated infra

[tannergooding]

LGTM.

[TonyValenti]

Regarding negative values, I THINK I have observed this in the past and attributed it to either a daylight savings time adjustment or windows time synchronization. Maybe those are no longer issues but I wanted to share.

[stephentoub]

It's worth reiterating that this special-casing was only in Stop. It didn't apply if you did:

sw.Start();
...;
TimeSpan elapsed = sw.Elapsed;

only if you did:

sw.Start();
...;
sw.Stop();
TimeSpan elapsed = sw.Elapsed;

That's true on .NET Framework as well.

Given that, even if there are still some machines in circulation where this could manifest, I'm comfortable removing this one place it was being special-cased.